Regulation of S-amino acids biosynthesis in Aspergillus nidulans. Role of cysteine and-or homocysteine as regulatory effectors.

Abstract

It was found that in Aspergillus nidulans the enzymes of the sulfate assimilation pathway and O-acetylhomoserine sulfhydrylase are under cysteine- and/or homocysteine, but not methionine- or S-adenosylmethionine-mediated regulation. These enzymes are repressed when the cells are grown in the presence of cysteine or homocysteine even in conditions where cysteine cannot be a precursor of homocysteine and vice versa. This was demonstrated by using mutants with impaired cystathionine cleavage enzymes. Thus, these two amino acids can substitute each other as the regulatory effectors. The addition of methionine causes repression only in conditions when it can be metabolized to homocysteine. The mutant cysA1 with a block at the serine transacetylase step is a prototroph owing to the existence of an alternative pathway for cysteine synthesis involving the enzymes: homocysteine synthase, cystathionine β-synthase and γ-cystathionase. All the three enzymes as well as those of the sulfate assimilation pathway are derepressed in this mutant. CysA1 mutation supresses the meth55 mutant blocked at β-cystathionase owing to the derepression of homocysteine synthase, so that the cystathionine cleavage step is bypassed. The results indicate that the pathway involving cystathionine formation is the main one for methionine biosynthesis in A. nidulans. The pathway involving homocysteine synthase is an alternative-conditional one, physiologically effective only when this enzyme is derepressed.